The present invention relates to a gain control circuit, which can be used in various parts of a low-noise amplifier used in a transmitter/receiver of mobile radio communication equipment, and which can operate as a variable gain circuit for compensating for the attenuation amount of a radio wave.
In general, in the field of radio communication, the attenuation amount of a radio wave changes with a communication distance. In mobile radio communication equipment such as mobile telephone, since the communication distance greatly changes, it has been required with a variable-gain amplifier circuit in order to compensate for the attenuation amount of the radio wave dependently upon the communication distance.
Referring to FIG. 10, there is shown a block diagram illustrating a fundamental construction of a transmitter/receiver section in a prior art mobile radio communication equipment. The shown transmitter/receiver section includes a receiver circuit part connected between one selection terminal of a switch 102 connected to an antenna 101 and one terminal of a base band processing part 113, the receiver circuit part including a low-noise amplifier 103, a band-pass filter (BP) 104, a mixer 108, an intermediate frequency (IF) amplifier 105, a band pass filter (BP) 106 and a mixer 108, which are cascade-connected in the named order. The shown transmitter/receiver section includes a transmitter circuit part connected between another terminal of the base band processing part 113 and the other selection terminal of the switch 102, the transmitter circuit part including a mixer 109, a band pass filter (BP) 112, a driver amplifier 111 and a power amplifier 110, which are cascade-connected in the named order.
In the above mentioned arrangement, the low-noise amplifier 103 and the intermediate frequency amplifier 105 in the receiver circuit part and the driver amplifier 111 in the transmitter circuit part can be constituted of the above mentioned variable-gain amplifier circuit.
Referring to FIG. 11, there is shown a circuit diagram illustrating a basic construction of the variable-gain amplifier circuit which can be used at various parts in the transmitter/receiver section of the mobile radio communication equipment. The shown variable-gain amplifier circuit is constructed to have a function similar to that of the variable-gain amplifier circuit disclosed in Japanese Patent Application No. Heisei 10-260734 published as JP-A-2000-091861. This variable-gain amplifier circuit includes an adjustable impedance circuit part including an NPN bipolar transistor 8 having a base connected to a control terminal 5 for receiving a controlled bias voltage, a collector connected to a power supply terminal 11 for receiving a positive fixed voltage, and an emitter connected through a resistor 6 to ground. The variable-gain amplifier circuit also includes a variable-gain amplifying circuit part having an amplifying circuit 1 having an input connected through a capacitor 9 to an input terminal 3 for receiving an input signal and an output connected to an output terminal 4, a node between the input terminal 3 and the capacitor 9 being connected to a node between the emitter of the transistor 8 and the resistor 6.
With this arrangement, the power supply terminal 11 connected to the collector of the transistor 8 is connected through a load (not shown) to a constant positive voltage power supply, and the control terminal 5 connected to the base of the transistor 8 is connected to a base bias circuit (not shown) which is formed of for example two bias resistors. In addition, as mentioned above, the emitter of the transistor 8 is connected through the resistor 6 to the ground. Thus, the adjustable impedance circuit part is constituted of the transistor 8 itself and the resistor 6. Since a constant positive voltage is applied to the collector of the transistor through the load from the power supply, the capacitor 9 functions as a DC blocking capacitor against the adjustable impedance circuit part when an amplified signal is outputted from the output terminal 4 of the amplifying circuit 1 in the variable-gain amplifier circuit.
In this variable-gain amplifier circuit, the shunt amount to the ground, of an input signal supplied to the input terminal 3, is controlled by the adjustable impedance circuit part, so that the amplification gain of the amplifying circuit 1 in the variable-gain amplifying circuit part can be varied. More specifically, since the control terminal 5 connected to the base of the transistor 8 is grounded in an AC mode, if the bias voltage applied to the control terminal 5 connected to the base of the transistor 8 is controlled, an impedance between the emitter of the transistor 8 and the ground (impedance of the transistor 8 and the resistor 6) changes, with the result that the shunt amount of the input signal supplied to the input terminal 3 is controlled, and therefore, the amplification gain of the amplifying circuit 1 changes.
As other known technique concerning the gain control circuit and the variable-gain amplifier circuit, a variable-gain amplifier circuit shown in JP-A-6-120756 and a gain control circuit shown in JP-A-2000-151311 are exemplified.
In the above mentioned gain control circuit (variable-gain amplifier circuit), if the resistance value of the resistor 6 connected to the emitter of the transistor in the adjustable impedance circuit part is small, the input signal is bypassed through the resistor directly to the ground, so that a SIN ratio (ratio of signal power to noise power) in the amplifying circuit lowers, with the result that a noise factor (called an xe2x80x9cNFxe2x80x9d) in a maximum gain condition becomes large. To the contrary, if the resistance value of the resistor 6 connected to the emitter of the transistor is large, the power supply voltage required for operating the transistor becomes large. Accordingly, if the resistance value of the resistor 6 is not set at an appropriate value, a stable operation cannot be obtained.
Specifically, in the gain control circuit shown in FIG. 11, assuming that the power supply voltage is 3.0V, a collector current of the transistor 8 in operation is 5.0 mA, a collector-emitter voltage drop of the transistor 8 is 1.0V, a maximum value Remax of the resistor 6 for allowing the transistor 8 to operate is expressed as the following equation (1):                               Re          max                =                                            3.0              -              1.0                                      5.0              xc3x97                              10                                  -                  3                                                              =                      400            ⁢                          xe2x80x83                        ⁢            Ω                                              (        1        )            
FIG. 12 illustrates a relation of the noise factor NF (dB) to the resistance value Re (ohm) of the resistor 6 in the maximum gain condition of this gain control circuit (resistance dependency of NF). Here, the characteristics of {ONLY RESISTOR} is referred to. For example, if it is desired to suppress the noise factor NF to not greater than 1.2 dB, the resistor is required to have the resistance of not less than 1.2 Kxcexa9, which is apparently larger than 400 xcexa9 as mentioned above. This means that the transistor 8 cannot operate under the power supply voltage of 3.0V.
Accordingly, it is an object of the present invention to provide a gain control circuit which has overcome the above mentioned problems of the prior art.
Another object of the present invention is to provide a gain control circuit allowing a variable-gain amplifier circuit to operate under a low power supply voltage with a lowered noise factor in the maximum gain condition.
A further object of the present invention is to provide a variable-gain amplifier circuit incorporating such a gain control circuit therein.
The above and other objects of the present invention are achieved in accordance with the present invention by a variable-gain amplifier circuit comprising;
an adjustable impedance circuit part including a bipolar transistor having a base connected to a control terminal for receiving a controlled bias voltage, one of a collector and an emitter of the bipolar transistor being connected to a first power supply terminal, and the other of the collector and the emitter of the bipolar transistor being connected to a second power supply terminal through a diode located in a forward direction to the second power supply terminal; and
a variable-gain amplifying circuit part having an amplifying circuit having an input coupled to an input terminal for receiving an input signal and an output connected to an output terminal,
the other of the collector and the emitter of the bipolar transistor connected to the second power supply terminal through the diode being connected through an DC blocking capacitor to the input of the amplifying circuit.
Preferably, the diode is connected in series with a resistor between the other of the collector and the emitter of the bipolar transistor and the second power supply terminal.
In a first embodiment of the variable-gain amplifier circuit, the first power supply terminal is a positive power supply voltage terminal and the second power supply terminal is connected to ground, the collector of the bipolar transistor being connected to the positive power supply voltage terminal, and the emitter of the bipolar transistor being connected to the ground through the diode connected in such a manner that an anode of the diode is connected to the emitter of the transistor and a cathode of the diode is connected to the ground. Furthermore, the input of the amplifying circuit is connected through the DC blocking capacitor to the input terminal, and the emitter of the bipolar transistor is connected to a node between the DC blocking capacitor and the input terminal.
In this first embodiment, a resistor can be connected in series with the diode between the emitter of the bipolar transistor and the ground.
In a second embodiment of the variable-gain amplifier circuit, the first power supply terminal is connected to ground and the second power supply terminal is a positive power supply voltage terminal, the emitter of the bipolar transistor being connected to the ground, and the collector of the bipolar transistor being connected to the positive power supply voltage terminal through the diode connected in such a manner that an anode of the diode is connected to the positive power supply voltage terminal and a cathode of the diode is connected to the collector of the bipolar transistor. Furthermore, the input of the amplifying circuit is connected through the DC blocking capacitor to the input terminal, and the collector of the bipolar transistor is connected to a node between the DC blocking capacitor and the input terminal.
In this second embodiment, a resistor can be connected in series with the diode between the collector of the bipolar transistor and the positive power supply voltage terminal.
In a third embodiment of the variable-gain amplifier circuit, the first power supply terminal is a positive power supply voltage terminal and the second power supply terminal is ground, the collector of the bipolar transistor being connected to the positive power supply voltage terminal, and the emitter of the bipolar transistor being connected to the ground through the diode connected in such a manner that an anode of the diode is connected to the emitter of the transistor and a cathode of the diode is connected to the ground. Furthermore, the input of the amplifying circuit is connected directly to the input terminal, and the emitter of the bipolar transistor is connected through the DC blocking capacitor to the input of the amplifying circuit.
In this third embodiment, a resistor can be connected in series with the diode between the emitter of the bipolar transistor and the ground.
In a fourth embodiment of the variable-gain amplifier circuit, the first power supply terminal is connected to ground and the second power supply terminal is a positive power supply voltage terminal, the emitter of the bipolar transistor being connected to the ground, and the collector of the bipolar transistor being connected to the positive power supply voltage terminal through the diode connected in such a manner that an anode of the diode is connected to the positive power supply voltage terminal and a cathode of the diode is connected to the collector of the bipolar transistor. Furthermore, the input of the amplifying circuit is connected directly to the input terminal, and the collector of the bipolar transistor is connected through the DC blocking capacitor to the input of the amplifying circuit.
In this fourth embodiment, a resistor can be connected in series with the diode between the collector of the bipolar transistor and the positive power supply voltage terminal.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.